Constant power regulator for xerographic fusing system

Abstract

A constant power regulator for a xerographic fuser in which power control is achieved by taking the sum of the load voltage and current. The regulator includes an operational amplifier connected as a voltage adding circuit. The operational circuit amplifier of the power regulator adds the voltage drop across the fuser and a reference resistor connected in series with the fuser and the voltage drop across the fixed reference resistance which represents the current flow through the fuser. The output of this summing circuit is detected by a photodetector that electrically isolates the power regulator from a voltage regulator which has an output for controlling the power supply to the fuser through, for example, a triac, controlled as a function of the power supply signal and the detected voltage generated by the power regulating circuit.

What is claimed is: 
     
       1. In a constant power regulator circuit for a variable impedance load which is coupled to a variable voltage alternating current power source, the improvement comprising: a summing circuit for maintaining substantially constant the power applied to said variable impedance load as a direct function of the voltage across said variable impedance load plus the current through said variable impedance load, wherein said summing circuit comprises;   a first voltage tap from said variable impedance load providing a first voltage signal proportional to the voltage applied across said variable impedance load,   a fixed current measurement impedance in series with said alternating current power source and said variable impedance load,   said fixed current measurement impedance having a second voltage tap providing a second voltage signal corresponding to the current through said variable impedance load,   an amplifier having a voltage responsive input,   first resistance means connecting said input of said amplifier with said first voltage tap to connect said first voltage signal to said amplifier input,   second resistance means connecting said second voltage tap to the same said input of said amplifier to connect said second voltage signal thereto and to sum said first and second voltage signals at said same input of said amplifier,   said first and second resistance means and said fixed current measurement impedance having values selected to nominally provide substantially equal first and second said voltage signals to said input of said amplifier,   said amplifier providing an output control signal controlled by said input; and   wherein zero crossing control means are connected between said alternating current power source and said variable impedance load to control the power applied to said variable impedance load, said control means being operative at zero crossing conditions of said alternating current power source,   said zero crossing control means being controlled by said output control signal from said amplifier of said summing circuit.